R: sourcing files using a relative path

Sourcing files using a relative path is useful when dealing with large codebases. Other programming languages have well-defined mechanisms for sourcing files using a path relative to the directory of the file being sourced into. An example is Ruby's require_relative. What is a good way to implement relative path sourcing in R?

Below is what I pieced together a while back using various recipes and R forum posts. It's worked well for me for straight development but is not robust. For example, it breaks when the files are loaded via the testthat library, specifically auto_test(). rscript_stack() returns character(0).